Food allergy is an increasingly prevalent disorder for which there are no cures. Innovative egies are needed to define its causes and identify targets for therapy. This project evaluates two novel hypotheses: 1) that mast cells activated by IgE antibodies, in addition to triggering anaphylactic reactions, play a separate and critical regulatory role as amplifiers of Th2 responses to food antigens and suppressors of Treg and 2) that specific IgG antibodies induced during antigen ingestion act on mast cells via Fc?RIIb to suppress their anaphylactic and immunoregulatory functions. We have developed a robust model of food allergy in which atopic Il4raF709 mice ingesting peanut or ovalbumin (OVA) exhibit strong IgE antibody production, Th2 responses and food anaphylaxis. We have found that mice lacking mast cells fail to develop allergic sensitization to ingested allergens, mounting protective Treg responses and producing inhibitory IgG antibodies which bind to Fc?RIIb and block IgE:FceRI signals. We hypothesize that mast cell cytokines and costimulatory molecules acting on intestinal DC, ILC2 and T cells drive Th2 responses and suppress Treg. Inhibitory IgG antibodies are hypothesized to counter these actions, dampening mast cell effects on T cell responses and altering mast cell homeostasis. Our hypotheses will be tested with the following aims: AIM 1: To evaluate mechanisms whereby mast cells promote Th2 induction and suppress Treg: 1.1 Examination of effects of mast cells on DC activation and induction of Th2 and Treg 1.2 Analysis of the effects of mast cell cytokines, including IL-33, on ILC2 1.3 Evaluation of the role of basophils in allergic sensitization in the intestine AIM 2: To test the impact of inhibitor IgG antibodies on regulation of food allergy: 2.1 Analysis of the effects of inhibitory IgG on mas cell cytokine production and control of Th2/Treg 2.2 Examination of effects of passively administered IgG on immune sensitization to foods 2.3 Evaluation of the effects of food specific IgG antibodies on intestinal mast cell homeostasis AIM 3: To test mechanisms of OIT in allergic subjects: 3.1 Tracking of peanut specific IgG and Treg induction in subjects undergoing OIT and analysis of the effects of IgE-blockade by omalizumab on these responses 3.2 Functional characterization of inhibitory IgG: identification of subclasses signaling via Fc?RIIb Anticipated outcome: By defining specific mechanisms whereby mast cell, IgE and inhibitory IgG antibodies regulate responses to food allergens these studies will identify critical pathways that ultimately lead to anaphylactic food sensitivity. This has the potential to lead to both novel diagnostic markers for food allergy and to the identification of candidate targets for therapy in established disease.